Generally, an image sensor is a semiconductor device that converts an optical image into electrical signals. The image sensor is classified into a Charge Coupled Device (CCD) image sensor, in which individual Metal Oxide Silicon (MOS) capacitors are located closely to each other such that charge carriers are stored in or discharged from the capacitors, and a CMOS image sensor employing a switching mode to sequentially detect an output by providing MOS transistors corresponding to the number of pixels through a CMOS technology that uses peripheral devices, such as a control circuit and a signal processing circuit.
A CMOS image sensor that converts information on an object into electrical signals includes signal processing chips having photodiodes, and an amplifier, an A/D converter, an internal voltage generator, a timing generator and a digital logic are combined in one chip. Accordingly, the CMOS image sensor is particularly advantageous in terms of space, power and cost reduction.
Hereinafter, details will be described regarding a circuit diagram showing the unit pixel of a 4T-type CMOS image sensor, which includes four transistors and one photodiode.
FIG. 1 is an equivalent circuit diagram showing a 4T-type CMOS image sensor. As illustrated in FIG. 1, unit pixel 10 of the 4T-type CMOS image sensor includes a floating diffusion node (FD) and a photodiode (PD) serving as a photoelectric conversion section and four transistors. The four transistors include a transfer transistor Tx, a reset transistor Rx, a drive transistor Dx, and a select transistor Sx.